Question about ROMs

I'm new to rooting and I still haven't tried any ROMs. I did make a backup right after I flashed clockwork with ROM Manager.

When you back up a ROM what exactly does it save? Is it like a carbon copy of everything or will you have to reinstall the apps.

Now I do understand that if I flash another ROM, I will have to probably wipe the data and cache. So obviously reinstall apps. Will I have to reactivate my phone if i do this? Will I have to reactivate my phone if I don't wipe data? Will I have to do it if I use my backup?

I know it takes less than a minute and its know big deal. I just want to know what to expect.

well its pretty simply really. Yes it backs everything up. So if youi revert back to it your phone will be all better. a lot of the roms now you dont have to wipe data or anything. I would try to instal before wipe and if you run into troubles then wipe. Most of the newer roms will instal and sinc back up on thier own. You will have to reenter your google password probably. SOmetimes depending on the rom you will need to activate phone but not if you dont wipe. A star 228 is a good idea anyway.